SUMMARY Alterations in cell metabolism support rapid growth and proliferation of cells - key hallmarks of cancer. Solute Carrier (SLC) transporters are membrane proteins that mediate the movement of solutes such as metabolites and drugs across membranes, and can also function cooperatively in cancer metabolism pathways. For exam- ple, the amino acid transporters LAT-1 and ASCT2 are highly upregulated in glioma, where they play a key role in supplying growing tumor cells with essential amino acids that are used as nutrients to build biomass and signaling molecules to enhance proliferation. Our broad goal is to understand cancer mechanisms, by de- scribing the substrate specificity determinants in selected cancer-related transporter families via an integrated experimental and computational approach. First, we will identify novel ligands including endogenous metabolites and prescription drugs for three cancer- related SLC transporters (i.e., GLUT1, ASCT2, and LAT-1), using comparative modeling, virtual screening, and chemoinformatics methods, followed by experimental validation (performed by collaborators). The results obtained in this Aim will reveal unknown functions of the transporters and provide novel chemical tools to further characterize their functions. Second, we will define rules for substrate specificity of structurally related transporters, using structural comparison methods and analysis of the predicted transporter-ligand complexes. The specificity determinants will be used to rationalize and predict the impact of genetic variation on trans- porter function. Third, we will develop methods to rationally design polypharmacological ligands for transport- ers in cancer-metabolism pathways and examine the anti-proliferative effect of the discovered ligands on glioblastoma multiforme (GBM) cell lines. We will further establish the role of the transporters and their ligands in cancer, by predicting previously unknown protein-small molecule interactions via chemical similarity methods. Finally, based on our results, we will rationalize differences and commonalities between metabolic pathways of normal and transformed cells. Characterizing the interactions between key membrane transporters and their small molecule ligands will con- tribute to our understanding of how solutes get transported across the membranes of cells and organelles, which is essential for many of life's processes. Furthermore, describing substrate specificity in proteins im- portant for cancer metabolism will provide chemical tools that would allow us to understand how perturbed metabolic networks are related to cancer and to discover novel leads for designing small molecules with opti- mized affinities against novel cancer drug targets. Ultimately, the results obtained in our studies can be a sig- nificant step toward designing cancer drugs.